The present invention is directed to carbon nanotubes in general and to direct synthesis of long strands or bundles of nanotubes by chemical vapor deposition.
There has been much interest in the production and processing of carbon nanotubes since their discovery. Aligned multi-walled carbon nanotubes with 2 mm length (see Z. Pan et al., Nature 394, 631 (1998)) and long single-walled nanotube (SWNT) ropes of up to three centimeters (see H. M. Cheng et al., Chem. Phys. Lett. 289, 602 (1998)) have been reported. Furthermore, macroscopic SWNT fibers and ribbons made by post-processing techniques (see B. Vigolo et al., Science 290, 1331 (2000)) have also been reported. However, the post processing techniques involve dispersing the nanotubes in surfactant solutions and then recondensing the nanotubes in a stream of polymer solution. This complicates the processing and affects the properties of the nanotubes. Thus, the creation of directly synthesized, continuous macroscopic strands of nanotubes during production still remains a challenge.
Chemical Vapor Deposition (CVD) is often used to produce single walled carbon nanotubes (SWNTs). CO gas was first used as a carbon source to synthesize single walled carbon nanotubes (see Dai et al., Chemical Physics Letters, 260, 471 (1996)). However, the SWNT yield was very low, and an extremely strict control of the CVD parameters was required. Subsequently, ethylene, methane, benzene were also used as carbon sources to produce single walled carbon nanotubes.
A floating catalyst CVD method with a horizontal furnace was used to synthesize single walled carbon nanotubes (see Cheng et al., Applied Physics Letters, 72, 3282 (1998)). The single walled carbon nanotubes were microscale in length, with a poor alignment and purity.
The floating catalyst method with a vertical furnace is an efficient method for mass-production of multiwalled carbon nanotubes and carbon nanofibers (see Ci, et al., Carbon, 38, 1933 (2000)). Benzene or xylene is usually used as a carbon source, ferrocene (Fe(C5H5)2) as a catalyst, and thiophene (C4H4S) as an additive. Typically ferrocene assisted CVD of hydrocarbons (benzene or xylene) produces multi-walled carbon nanotubes at lower temperatures (about 1050 K) and mixture of single-walled and multi-walled nanotubes at higher temperatures (>1300 K) (see M. Endo et al., J. Phys. Chem. Solid 54, 1841 (1993); R. Andrews et al., Chem. Phys. Lett. 303, 467 (1999); H. M. Cheng et al., Appl. Phys. Lett. 72, 3282 (1998)). However, benzene is toxic and might pollute the environment. The addition of thiophene has been shown to increase the yield of single-walled nanotubes (see H. M. Cheng et al., Appl. Phys. Lett. 72, 3282 (1998)).